A conventional shock sensor used in a data storage device (DSD), for example a hard disk drive (HDD), to detect mechanical shocks and prevent off-track writes (OTWs) may be designed as a cantilever beam structure with piezoelectric properties. A shock event causes the beam to deflect producing a small electric charge. The small electric charge produced by the shock sensor is amplified and filtered by a shock detection system and the output of the shock detection system is compared to a predetermined voltage threshold to determine whether the detected shock is large enough to require suspension of write operations. When the output of the shock detection system exceeds the predetermined voltage threshold, an interrupt signal is issued to the system-on-a-chip (SoC), which immediately suspends write operations to prevent overwriting data on adjoining tracks.
FIG. 1 is a diagram illustrating a DSD 100 including a conventional shock detection system. Referring to FIG. 1, the DSD 100 may include a disk 110 rotated by a spindle 115 coupled to a spindle motor 120 and a head 125 connected to an end of an actuator arm 130 which is rotated about a pivot by a voice coil motor (VCM) 135 to position the head 125 radially over the disk 110. The VCM 135 may controlled by a VCM drive signal 157 from a VCM control circuit 155. The disk may include a number of concentric data tracks each partitioned into a number of data sectors. The spindle motor 120 may be driven by a spindle drive signal 142 generated by a pulse width modulator (PWM) 140. A control unit 150 may control the PWM 140 and the VCM control circuit 155 and may receive input from a shock detection system 160.
FIG. 2A is a simplified block diagram of a shock detection system in accordance with various aspects of the present disclosure. The shock detection system 160 may include a shock sensor 165, a first gain stage (e.g., a charge amplifier) 170, a notch filter 175, additional circuitry including filters 180, for example, but not limited to, firmware tunable filters, and gain stages 185, and one or more window comparators 190. The shock detection system 160 may be configured to generate an interrupt signal to the control unit 150 upon detection of a mechanical shock exceeding a threshold.
Since the shock sensor 165 is typically designed as an under-damped cantilever beam structure (although other configurations can also be used), the mechanical response of the shock sensor 165 may be that of a second-order mechanical system with a pronounced resonance, usually with a Q-factor on the order of 50. As a result, any electrical noise, mechanical vibration, or other disturbances near the resonance frequency of the shock sensor 165 will be amplified and may cause the output signal 162 of the shock detection system 160 to exceed the predetermined voltage threshold. Therefore, a notch filter 175 may be used as part of the shock detection system 160 to suppress the resonance of the shock sensor 165 while minimizing the phase delay of the shock detection system 160 at lower frequencies to provide rapid detection of shock events.
FIG. 3 is a graph 300 illustrating example gain and phase plots of the shock detection system before and after application of a conventional fixed notch filter for a shock detection system output signal 162. Referring to FIG. 3, the example gain 310 and phase 320 plots of the shock detection system without a notch filter and the example gain 330 and phase 340 plots of the shock detection system output signal 162 after application of a conventional notch filter with a Q-factor of 1.0 when frequency of the notch filter (fnotch) is equal to the resonance frequency of the shock sensor (fsensor) are shown.
To prevent OTWs due to external shocks and thermal pops (i.e., small, high-frequency shock events caused by mismatch of the thermal coefficients of expansion of the materials inside the DSD 100), maximum gain and minimal phase delay is required up to about 20 kHz. As illustrated by the gain 330 and phase 340 plots in FIG. 3, a notch filter Q-factor of 1.0 sufficiently suppresses the resonance frequency of the sensor while maintaining a flat output magnitude and minimizing the phase loss.
However, low-cost shock sensors typically used in DSDs have a wide part-to-part variation (approximately +/−20%) in actual resonance frequency. Therefore, if a fixed notch filter frequency is used, the Q-factor of the filter would have to be low which may result in a significant phase loss and large delay in responding to shocks.
FIG. 4 is a graph 400 illustrating example gain and phase plots before and after application of a conventional fixed notch filter for a shock detection system output signal 162. In FIG. 4, example gain 410 and phase 420 plots of the shock detection system output signal 162 without a notch filter and example gain 430 and phase 440 plots of the shock detection system output after a conventional notch filter with a Q-factor of 0.25 when frequency of the notch filter (fnotch) is not equal to the resonance frequency of the shock sensor (fsensor) are shown. Referring to FIG. 4, the specified resonance frequency range of a typical shock sensor may be 44±8 kHz. With this wide range of part-to-part variation in resonance frequency (and without resonance detection), the Q-factor would need to be set to 0.25 to achieve sufficient resonance suppression (20 dB), leading to a large phase loss.